Should i be concerned if my chiropractor said the bottom part of my back is curving?

That depends on what: Is curving and it what direction as well as what symptoms you are experiencing along what may or may not be present on your clinical or physical examination. If you are not improving or questioning your treatment or diagnosis, see an orthopaedic surgeon, physiatrist, neurosurgeon or a neurologist for another opinion or start with your family physician who can refer you out if needed.